opendir (DIR, "/home/costas/test") or die "cannot opendir dir";
foreach $file (readdir(DIR)) 
{
    rename($file, lc($file));
[download]

That doesn't work, because you get only the filenames from readdir and not their paths. You will have to prefix "/home/costas/test/" yourself:

my $path = '/home/costas/test';

opendir (DIR, $path) or die "Can't opendir: $!";

for (readdir DIR) {
    rename("$path/$_", "$path/" . lc);
}

closedir DIR;
[download]

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